These are refrigerator pickles, so no heat processing is used. Fill the jars (I do love a Weck jar...) as tightly as possible with the pickles and onions, as they will shrink some as they pickle. This recipe is easily doubled, tripled, you name it.
In a quart measuring cup or bowl, combine the vinegar and granulated sugar. Whisk occasionally, until the sugar is completely dissolved. Add the mustard seeds.
Trim the ends of the cucumbers and discard. Slice into 1/4- to 1/2-inch rounds. Trim and slice the onion into 1/4- to 1/2-inch half-moon slices. Toss the cucumbers and onions in a bowl to combine.
Pack the cucumbers and onions as tightly as possible in clean jars. Pour the pickling liquid over all in both jars, spooning the mustard seeds in to evenly distribute them.
Close the lids and refrigerate for about two days before serving. Pickles will stay nice in the refrigerator in their pickling liquid for a few weeks.